Nerves

Anterolateral Abdominal Wall Nerves

The anterior rami of the lower six thoracic and the first lumbar nerves supply the anterolateral abdominal wall (see Figs. 6.5, 6.6, and 6.20). The thoracic nerves are the continuations of the lower five intercostal nerves and the subcostal nerves beyond the costal margins (see Fig. 6.6). They pass forward in the interval between the internal oblique and the transversus muscles in the same way the intercostal nerves run forward in the intercostal spaces between the internal intercostal and the innermost intercostal muscles. Collectively, they supply the skin of the anterolateral abdominal wall, the muscles, and the parietal peritoneum. The lower six thoracic nerves pierce the posterior wall of the rectus sheath to supply the rectus muscle and the pyramidalis (T12 only). They terminate by piercing the anterior wall of the sheath and supplying the skin.

The first lumbar nerve, a branch of the lumbar plexus, has a similar course as the thoracic nerves, but does not enter the rectus sheath (see Fig. 6.20). It divides into the iliohypogastric and ilioinguinal nerves (see Figs. 6.5 and 6.6). The iliohypogastric nerve pierces the external oblique aponeurosis above the superficial inguinal ring, and the ilioinguinal nerve emerges through the ring. They end by supplying the skin just above the inguinal ligament and symphysis pubis.

The dermatomes of the anterolateral abdominal wall are shown in Figure 6.5. Useful landmark references are that the dermatome of T7 is located in the epigastrium over the xiphoid process, that of T10 includes the umbilicus, and that of L1 lies just above the inguinal ligament and the symphysis pubis.

More simply put: Dermatome T7: xiphoid process Dermatome T10: umbilicus Dermatome L1: pubis

Muscle Rigidity and Referred Pain

Sometimes, a clinician has difficulty in determining whether the muscles of the anterior abdominal wall of a patient are rigid because of underlying inflammation of the parietal peritoneum or whether the patient is voluntarily contracting the muscles because he or she resents being examined or because the clinician’s hand is cold. Asking the patient, who is lying supine on the examination table, to rest the arms by the sides and draw up the knees to flex the hip joints, usually easily solves this problem. It is practically impossible for a patient to keep the abdominal musculature tensed when the thighs are flexed. Needless to say, the examiner’s hand should be warm.

A pleurisy involving the lower costal parietal pleura causes pain in the overlying skin that may radiate down into the abdomen. Although this is unlikely to cause rigidity of the abdominal muscles, it may cause confusion in making a diagnosis.

Anterior Abdominal Nerve Block

An anterior abdominal nerve block is performed to repair lacerations of the anterior abdominal wall.

Area of Anesthesia

The area of anesthesia is the skin of the anterolateral abdominal wall. The relevant nerves are the anterior rami of the 7th through the 12th thoracic nerves and the first lumbar nerve (ilioinguinal and iliohypogastric nerves).

Procedure

The anterior ends of intercostal nerves T7 through T11 enter the abdominal wall by passing posterior to the costal cartilages (see Figs. 6.5 and 6.6). An abdominal field block is most easily carried out along the lower border of the costal margin and then infiltrating the nerves as they emerge between the xiphoid process and the 10th or 11th rib along the costal margin (Fig. 6.22).

The ilioinguinal nerve passes forward in the inguinal canal and emerges through the superficial inguinal ring. The iliohypogastric nerve passes forward around the abdominal wall and pierces the external oblique aponeurosis above the superficial inguinal ring. Blocking the two nerves is easily accomplished by inserting the anesthetic needle 1 in. (2.5 cm) above the anterior superior iliac spine on the spinoumbilical line.

Figure 6.22 Anterior abdominal wall nerve blocks. T7 through T11 (A) are blocked (X in B) as they emerge from beneath the costal margin. The iliohypogastric and ilioinguinal nerves (A) are blocked by inserting the needle about 1 in. (2.5 cm) above the anterior superior iliac spine on the spinoumbilical line (X in B). The terminal branches of the genitofemoral nerve (A) are blocked by inserting the needle through the skin just lateral to the pubic tubercle and infiltrating the subcutaneous tissue with anesthetic solution (X in B).